Creeks have a tendency to erode at the banks over time, sometimes exposing dump sites. They were also a place of choice for dumping in the first place, as the farmers or land owners could do little else with a creek bank. This doesn't mean the bottles will be any better than a "land dump" but the locating and subsequent excavating can be more difficult, also, these land dumps were often burned.. that probably didn't happen so much with creek-side dumps, because there was more vegetation around them.. now if you are exploring a densely populated area like where I grew up, creeks were pretty much the only place to dump, and very easy bottle-picking, as long as you were a cute little kid who didn't get the cops called on him.. a "buzz off!" got rid of me! []
I can only say to do what I did, and still do from time to time.. get a pair of rubber boots, go find a creek, and walk it up and down... you know what to look for!
That's the way I found the best creek dump I ever dug was noticing rust color in the earth.
Metal, wire, car parts, glass bits are what you look for to locate a creekbank dump.